Vehicle manufacturers are developing hybrid vehicles to meet the demand for more fuel efficient vehicles. One configuration for a hybrid vehicle may be referred to as a Modular Hybrid Transmission (MHT) vehicle design. A MHT offers substantial fuel economy improvement by using electric drive for vehicle propulsion and regenerative braking. In a MHT vehicle, a motor is operatively connected between a conventional automatic step ratio transmission and the engine. The motor is attached to the transmission impeller or input shaft. The engine is selectively disconnected from the traction motor and the transmission by a “disconnect clutch”. The disconnect clutch allows a vehicle to be driven in an electric mode using only the traction motor, in a hybrid mode with both motor and the engine propelling the vehicle, or in a conventional mode with the vehicle being propelled by the engine only.
Turbocharged engines are used to provide improved fuel economy while improving acceleration and drivability. Turbocharged engines allow designers to reduce engine size while still maintaining acceptable power for vehicle launch by boosting intake manifold pressure. Conventional turbochargers are known to have “turbo lag” until the turbocharger achieves a certain minimum speed to develop increased intake manifold pressure or boost. The engine can only obtain the minimum speed with sufficient exhaust gas flow that boosts the intake manifold pressure. Turbo lag is very pronounced at high altitudes because reduced barometric pressure and reduced oxygen concentration increases the amount of time for the exhaust gas pressure to build. Turbo lag also increases if the vehicle is carrying or towing a large load, such as when the vehicle is towing a trailer. Road grade may also contribute to turbo lag, such as when the vehicle is climbing a hill.
Turbo lag becomes an even bigger problem if the engine drives an MHT vehicle having a torque converter. Torque converters have a “stall speed” at which the converter input or impeller must spin for the torque converter to develop sufficient torque to move the vehicle. “Loose torque converters” are used to allow for high stall speeds and associated torque multiplication to facilitate vehicle launch at high altitudes. However, a loose torque converter incurs a fuel economy penalty and limits engine downsizing. Turbo lag can be exacerbated to the point where at high altitude it can take several seconds for a vehicle having a small turbocharged engine to accelerate even on level grade surfaces. Turbo lag at high altitudes becomes even greater if the vehicle is moving uphill or pulling a trailer.